Update: Microsoft has now changed the license to the
MEF to the open source MS-PL license. Please see
the follow
up post for details.

As a .NET developer, you should avoid using the newly
released Managed
Extensibility Framework
as its
license prevents its use beyond the Windows platform.
This will prevent your .NET software from running on Linux or
MacOS in the future.

Luckily, there is a cross platform solution available today
that has no platform
limitations: Mono.Addins,
a technology inspired by Eclipse's own plugin system. We
have tutorials,
reference
manuals, API
docs, our FAQ
our public
groups and multiple large applications with source code
available that you can learn from (MonoDevelop, Banshee,
F-Spot and Gnome-Do).

The rule obviously applies to any new APIs that are built
for .NET as they are not immediately available for Mono. But
unlike the binary-only APIs, these half-open source code
releases pose additional problems for the open source CLI:

More people are exposed to the source code,
preventing them from working on a fully open source
implementation of it.

There is a smaller desire from the community to
reimplement the code, as it is relatively easy to just
use the existing implementation or ignore the issue
for the time being.

Some folks might not care about the license
restriction, and ignore it altogether. A practice I
do not endorse.

There are two additional issues that are worth pointing
out. Should non-open source libraries be hosted
on CodePlex to begin
with? CodePlex is branded as the "Open Source Project
Hosting" from Microsoft, yet, this license is clearly
not open source and does not qualify as open
source according
to the definition and is a clear violation of
Codeplex.Com's requirements:

MEF should be pulled out of the CodePlex site along
with all other platform-limiting software, like ASP.NET MVC
(h/t to gblock for pointing out that MVC is also violating the
terms). Unless CodePlex cooks up a special exception that some
software and restrictions are more equal than others.

The second point that is worth making is that picking
licenses like the MS-LPL for .NET software is shooting the
.NET community in the foot. The LPL license is obviously an
effort to tie things into the Windows platform, putting
company first, community and developers second.

The MS-LPL is a poisonous license, do not use it (do not
confuse with the MS-PL which is a decent license, the extra
"L" makes a big difference).

Update: gblock points out that CodePlex technically
only requires a "license" to be picked, but does not require
it to be OSI compliant. But even if the wording on the FAQ
does allow for that interpretation, it is misleading as there
is a link to OSI licenses next to it, and the main page
clearly states that Codeplex is an "Open Source Project
Hosting" site. Capital "O" and capital "S".